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The Purpose of the extended Idle was in police vehicles when sitting still using extra accessories to prevent too much strain on the motor. For post 93 jeeps there are a plethora of how to pages on how to add this feature Why would you want extended Idle or a higher idle? well many reasons, winch recovery, lights, On board air, overheating (assume good clutch fan) After several hours of research all that I've found for doing the extended idle was for post 93 vehicles where a police version was available. After reading pages and pages of forums discussing the post 93 and people just begging for how to do it on pre 93 vehicles but I was unsuccessful in finding it. I acquired a 90fsm and searched it my self and found no direct options listed. But then Blue88Comanche suggested piggybacking the up idle from the ac circuit. Yeah it's not a true extended idle and the idle isn't increasing to 1k rpm like the extended idle but 300-400 rpm increase is plenty for my purposes. Again Back to the wire diagrams I went.... What I found: There are two inputs to the ecu for the A/C. One from the physical switch giving power to the a/c circuit then a second that comes off the a/c thermo. The ECU looks for voltage on these two contacts then increases the Idle expecting the draw from the ACpump. Connections c2 and d2. An attempt to further explain the a/c electrical: when a/c is selected power goes to the low pressure switch at the condenser, comes back into cab and is split; one of these splits goes to the ecu at d2. A second split goes to the a/c thermo then runs back to ecu at c2. a third leg supply power at the a/c relay for both the aux fan and the compressor, but this power is controlled through a relay by the output of ecu a12. when c2=d2(12volts) idle increases expecting the load and at the same time a12 powers the relay in engine compartment closed and the compressor is activated with the fan. I don't have A/C So I was not concerned with what I did. And alot of the A/C circuitry is not there just stuff in the factory loom... If you have a/c and want to retain the use of a/c a few relays maybe needed to break different circuits, and probably cutting and tapping into wire as they enter the ecu rather than tying into them down stream in the harness like i did. Wire diagrams referenced are from the 90 fsm 22,74, 75, 76 I did this to an 89 4.0 although based on the wire diagrams for the 2.5 it should be the same too, but different pins on the ecu, connector behind the fan switch listed below is the same though. The HOw: It's pretty simple, Remove dash bezel, remove stereo, loosen the heat controller. Look for this six pin plug behind the fan controller. Mine had 5 wires on the female side and only 2 or 3 on the male side Decide your location for your high idle switch and your 12 volt source (I used a tap off cig lighter as i already have a modified circuit controlling that power). You'll run the switched power to the connector and connect it to pins a and b a light green wire and a light brown wire. I got two jeep connectors from a male donor plug and wired it in the the male side of the factory plug so it's clean: (this is a soldered connection heat shrunk with dual wall then I wrapped with electrical tape for stress, anal or overkill you choose) I fed these contacts off a "fan diode assembly" that was a donor from the jy with a/c. There are three connections on these the two outsides are source in and the middle is output. They do not allow back feed and are rated to 1.5 amp (since everything is on relays and just the ecu no worries) anyways on one leg I tied my manual aux fan switch and the other leg to my future elocker switch. When I use these I clean them, and change the gray and orange wire so that the two inputs are the same color, when this is in the factory position the grey wire comes from the thermostat in the radiator, the orange "in" is from the a/c circuit and the orange "out" controls the fan relay. nice and finished and works flawlessly: